Suppression of interference at the wireless receiver is an important component of current and developing communication systems. Interference cancellation performance affects transmission power requirements and link utilization efficiencies in both the uplinks and downlinks of wireless communication systems, such as cellular communication networks based on the Wideband Code Division Multiple Access (WCDMA) or IS-2000 standards. Better interference cancellation enables data transmission at lower power levels and/or at higher data rates than would otherwise be possible.
The particulars of interference cancellation vary as a function of many variables, such as the communication signal types and protocols involved, details of the transmitting and receiving equipment, etc. However, providing good interference cancellation performance generally requires significant signal processing resources, because of the need to characterize and suppress received signal interference in real time.
For example, the well known generalized Rake (G-Rake) receiver uses extra de-spreading fingers to suppress interference and improve demodulation. Impairment cross-correlations between the fingers can be represented as an impairment covariance matrix Ru; that matrix in turn can be used to generate the combining weights used by the G-Rake receiver in combining de-spread data values. By computing the combining weight vector, w, as:w=Ru−1h,  Eq. (1)the G-Rake receiver uses the impairment covariance matrix to whiten colored interference in the received signal(s) of interest. In the above expression, h is the net channel response vector; each element of h represents the overall propagation channel response between a signal transmitter and a receiver finger, including the radio channel as well as the transmitter and receiver pulse-shaping filters.
There are several approaches for generating Ru in the G-Rake context. For example, the parametric G-Rake receiver models the impairments as a sum or combination of different interference contributions, including contributions from own-cell interference, white noise, and other-cell interference. With this parametric model, and assuming the reception of signals from J+1 network base stations, the impairment covariance matrix is given by:
                                                                                          R                  u                                =                                ⁢                                                                            E                      c                                        ⁢                                          R                      I                                                        +                                                            N                      0                                        ⁢                                          R                      n                                                        +                                                            ∑                                              j                        =                        1                                            J                                        ⁢                                                                  E                        c                        j                                            ⁢                                              R                        O                        j                                                                                                                                                                    =                                ⁢                                                                            N                      0                                        ⁢                                          R                      n                                                        +                                                            ∑                                              j                        =                        0                                            J                                        ⁢                                                                  E                        c                        j                                            ⁢                                              R                        O                        j                                                                              -                                      hh                    H                                                                                      ,                            Eq        .                                  ⁢                  (          2          )                    where Ec is the average energy transmitted per chip of own-cell base station, N0 is the one-sided power spectral density of white noise, Ecj is the average energy transmitted per chip by the j-th other-cell base station, RI is a covariance matrix modeling own-cell interference, Rn models white noise passed through pulse shaping filter, and ROj is a covariance matrix modeling other-cell interference from the j-th other cell. Those skilled in the art will appreciate that the second formulation of Eq. (2) demonstrates that the own-cell interference term can be computed in a fashion similar to that used for other-cell interference, provided that a benign signal term is subtracted—e.g., RI=RO0−hhH.
In practical receiver implementations where other-cell interference is not explicitly modeled, i.e., where other-cell interference ROj is modeled as white noise, the computation of RI contributes significantly to the overall complexity of a parametric G-Rake receiver operation. In other words, improving the efficiency of the computation of RI reduces the signal-processing complexity of a parametric G-Rake receiver, freeing computation resources for other receiver tasks. In receivers where other-cell interference (ROj) is explicitly modeled, the calculation of the other-cell interference matrices similarly affects the complexity of the receiver. Improving the efficiency of these calculations is thus similarly advantageous.
One approach for calculating entries RI (d1,d2) for RI is based on the equation:
                                                        R              I                        ⁡                          (                                                d                  1                                ,                                  d                  2                                            )                                =                                    ∑                              l                =                0                                            L                -                1                                      ⁢                                          ∑                                  q                  =                  0                                                  L                  -                  1                                            ⁢                                                g                  l                                ⁢                                  g                  q                  *                                ⁢                                                      ∑                                                                  m                        =                                                  -                          ∞                                                                    ,                                              m                        ≠                        0                                                                                    m                      =                      ∞                                                        ⁢                                                                                    R                        p                                            ⁡                                              (                                                                              d                            1                                                    -                                                      mT                            c                                                    -                                                      τ                            l                                                                          )                                                              ⁢                                                                  R                        p                        *                                            ⁡                                              (                                                                              d                            2                                                    -                                                      mT                            c                                                    -                                                      τ                            q                                                                          )                                                                                                                                ,                            Eq        .                                  ⁢                  (          3          )                    where gl represents the l-th medium channel coefficient, dk is the k-th finger delay, τj is the j-th channel delay, Tc is a CDMA chip duration, and Rp(*) is the autocorrelation of the receive pulse shaping filter. (Note that if the transmit and receive pulse filters are not the same, then Rp(*) includes convolution of the transmit and receive pulse filters.) The medium channel coefficients gl represent the response of the radio channel alone, unlike net channel coefficients, which include the response of the transmit and receive filters.
Eq. (3) may be shown to be equivalent to:
                                                        R              I                        ⁡                          (                                                d                  1                                ,                                  d                  2                                            )                                =                    ⁢                                    ∑                              l                =                0                                            L                -                1                                      ⁢                                          ∑                                  q                  =                  0                                                  L                  -                  1                                            ⁢                                                g                  l                                ⁢                                                      g                    q                    *                                    ⁡                                      [                                                                                            R                          pp                                                ⁡                                                  (                                                                                    Δ                              1                                                        -                                                          Δ                              2                                                                                )                                                                    -                                                                                                    R                            p                                                    ⁡                                                      (                                                          Δ                              1                                                        )                                                                          ⁢                                                                              R                            p                            *                                                    ⁡                                                      (                                                          Δ                              2                                                        )                                                                                                                ]                                                                                      ,                            Eq        .                                  ⁢                  (          4          )                                        where                                                                                      ⁢                                            Δ              1                        =                        ⁢                                                            d                  1                                -                                                      τ                    l                                    ⁢                                                                          ⁢                  and                  ⁢                                                                          ⁢                                      Δ                    2                                                              =                                                d                  2                                -                                  τ                  q                                                              ,          and                                                                                          R            pp                    ⁡                      (                                          Δ                1                            -                              Δ                2                                      )                          =                              ∑                          m              =                              -                ∞                                      ∞                    ⁢                                                    R                p                            ⁡                              (                                                      Δ                    1                                    -                                      mT                    c                                                  )                                      ⁢                                                            R                  p                  *                                ⁡                                  (                                                            Δ                      2                                        -                                          mT                      c                                                        )                                            .                                                                      Rpp(*) can be pre-computed, thereby saving run-time computations, such that the signal processing implementation of Eq. (4) reduces to a few table lookups and a multiplication. However, the table lookup for Rpp(*) is somewhat complicated because it depends not only on the difference between Δ1 and Δ2, but also at what sample phase the difference occurs. Also, because Rpp(*) is not symmetric, one must also account for positive and negative delay differences in selecting lookup table entries.
Because of the above complications, the required table lookup operations depend on a greater number of variables, meaning more processing decisions have to be made to identify proper table entries, which in turn requires greater processing power or speed and a greater amount of working memory. Such complications detract from the efficiency gains otherwise afforded by the implementation of Eq. (4) for RI computation in a parametric G-Rake receiver.
Another approach expresses RI as:
                                          R            I                    ⁡                      (                                          d                1                            ,                              d                2                                      )                          =                              ∑                          l              =              0                                      L              -              1                                ⁢                                    ∑                              q                =                0                                            L                -                1                                      ⁢                                          g                l                            ⁢                                                                    g                    q                    *                                    [                                                            R                      ⁡                                              (                                                                              n                            1                                                    ,                                                      n                            2                                                                          )                                                              -                                                                  1                                                  N                          2                                                                    ⁢                                                                        ∑                                                      m                            =                                                          1                              -                              N                                                                                                            N                            -                            1                                                                          ⁢                                                                              (                                                          N                              -                                                                                              m                                                                                                                      )                                                    ⁢                                                                                    R                              p                                                        ⁡                                                          (                                                                                                d                                  1                                                                -                                                                  mT                                  c                                                                -                                                                  τ                                  l                                                                                            )                                                                                ⁢                                                                                    R                              p                              *                                                        ⁡                                                          (                                                                                                d                                  2                                                                -                                                                  mT                                  c                                                                -                                                                  τ                                  q                                                                                            )                                                                                                                                                            ]                                .                                                                        Eq        .                                  ⁢                  (          5          )                    Eq. (5) has similarities to the formulation given in Eq. (4), with R(n1,n2) equivalent to (and replacing) Rpp(*), but includes a more complicated expression involving the sum of products of Rp(*). Under limited reception conditions—i.e., minimal time dispersion in the propagation channel—the lookup tables for R(n1, n2) and Rp(*) need only span a few CDMA spreading chips to yield acceptable performance. However, Eq. (5) does not necessarily yield good performance over a range of channel conditions, and still entails significant computational complexity.
In a related U.S. patent application Ser. No. 11/479,483, titled “Method and Apparatus for Interference Estimation in a Generalized Rake Receiver” to D. Cairns and G. Bottomley (the “Cairns” application), the entire contents of which are incorporated by reference herein, another method for determining impairment correlations between a plurality of delays of interest for a received CDMA signal was disclosed. First, it was shown that the entries of the impairment covariance matrix given by Eq. (3) can be re-written as:
                                          R            I                    ⁡                      (                                          d                1                            ,                              d                2                                      )                          =                              ∑                                          m                =                                  -                  ∞                                                            m                ≠                0                                      ∞                    ⁢                                    (                                                ∑                                      ℓ                    =                    0                                                        L                    -                    1                                                  ⁢                                                      g                    ℓ                                    ⁢                                                            R                      p                                        ⁡                                          (                                                                        d                          1                                                -                                                  mT                          c                                                -                                                  τ                          ℓ                                                                    )                                                                                  )                        ⁢                                                            (                                                            ∑                                              q                        =                        0                                                                    L                        -                        1                                                              ⁢                                                                  g                        q                                            ⁢                                                                        R                          p                                                ⁡                                                  (                                                                                    d                              2                                                        -                                                          mT                              c                                                        -                                                          τ                              q                                                                                )                                                                                                      )                                *                            .                                                          Eq        .                                  ⁢                  (          6          )                    If the first summation is modified to include the m=0 term, then Eq. (6) can be re-written as:
                                          R            I                    ⁡                      (                                          d                1                            ,                              d                2                                      )                          =                                            ∑                              m                =                                  -                  ∞                                            ∞                        ⁢                                          (                                                      ∑                                          ℓ                      =                      0                                                              L                      -                      1                                                        ⁢                                                            g                      ℓ                                        ⁢                                                                  R                        p                                            ⁡                                              (                                                                              d                            1                                                    -                                                      mT                            c                                                    -                                                      τ                            ℓ                                                                          )                                                                                            )                            ⁢                                                (                                                            ∑                                              q                        =                        0                                                                    L                        -                        1                                                              ⁢                                                                  g                        q                                            ⁢                                                                        R                          p                                                ⁡                                                  (                                                                                    d                              2                                                        -                                                          mT                              c                                                        -                                                          τ                              q                                                                                )                                                                                                      )                                *                                              -                                    h              ⁡                              (                                  d                  1                                )                                      ⁢                                                            h                  ⁡                                      (                                          d                      2                                        )                                                  *                            .                                                          Eq        .                                  ⁢                  (          7          )                    The right-most term, h(d1)h(d2)*, is readily calculated from measured net channel response data. Efficient methods for calculating the remaining term were presented in the Cairns application, including time-domain convolution techniques (and the frequency-domain equivalents) for computing the impairment matrix terms.
Those skilled in the art will appreciate that each of the above impairment matrix formulations are written in terms of medium coefficients, i.e., g, which are the radio channel coefficients. In a practical receiver implementation, estimated medium coefficients are typically obtained by estimating the net coefficients, and then applying some variant of a transformation:g=(BHB)−1BHh,  Eq. (8)where B is a conversion matrix. The (i,j) element of B is given by:bi,j=RTX/RX(di−τj).  Eq. (9)Here, RTX/RX(λ) is the convolution of the transmit and receive filters evaluated at λ (di and τj were defined above in connection with Eq. (3)).
In practice, optimal performance of interference-suppressing receivers based on any of the above impairment estimation techniques may depend on precise knowledge of path delays (τj) under some circumstances, especially when processing signals with a high signal-to-interference-plus-noise ratio (SINR). Inaccurate path delay information results in correspondingly inaccurate estimates of the medium response coefficients g. In some cases, an inaccurate estimate of the medium response may result in non-optimal receiver performance, even if the interference estimation process is very efficient.